Parex unit feed

ABSTRACT

A process for separating a product from a multicomponent feedstream to an adsorption apparatus or system. The apparatus or system may comprise a moving-bed or a simulated moving-bed adsorption means. The product comprises at least one organic compound, such as an aryl compound with alkyl substitutes. In embodiments the conduits used to supply the feedstream to the apparatus or system are flushed with media of multiple grades. In embodiments the process achieves improvements in one or more of efficiency of adsorption separation, capacity of adsorption apparatus systems, and purity of product attainable by adsorption process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Application No.61/182,481, filed May 29, 2009, the disclosures of which areincorporated by reference in their entireties.

FIELD OF THE INVENTION

The invention relates to a process for separating one or more of thecomponents from two or more multicomponent fluid mixtures, and moreparticularly to a process for separating organic compounds from such afluid mixture by means of adsorption apparatus, such as moving-bed orsimulated moving-bed adsorption apparatus, or a system comprising suchapparatus.

BACKGROUND OF THE INVENTION

Various means are currently available to separate the components of amulticomponent fluid mixture. If the densities of the components differsufficiently, the effects of gravity over time may be adequate toseparate the components. Depending on the quantities of the componentsinvolved, a centrifuge may be used to more rapidly separate componentswith different densities. Alternatively, distillation may be used toseparate components with different boiling points.

Some fluid mixtures comprise components which have similar boilingpoints, and in such cases, separation by distillation may be a difficultand an inefficient means to separate these components. Too manycontaminants, e. g., unwanted components, also may evaporate along with(or fail to evaporate from) the desired component(s), or the separationmay require high energy expenditures due to the recycling through thedistillation process that may be necessary to attain a desired degree ofseparation or purity.

In view of these and other deficiencies of these aforementionedprocesses, adsorption often has been preferred as a process forseparating the components from a multicomponent fluid mixture to obtainrelatively pure products.

The efficiency of an adsorption process may be partially dependent uponthe amount of the surface area of the adsorbent solids which isavailable for contact with a fluid mixture. The surface area availablemay be more than just the superficial, external surface of the solids.Suitable solids also may have internal spaces. Such internal spaces maycomprise pores, channels, or holes in the surface of the solids and mayrun throughout the solids, much as in sponges. Thus, the fluid contactsnot only the superficial surface, but penetrates into the solids. Sievechambers increase the contact surface between the fluid and the solidsin an adsorption process by concentrating them in a confined space. Suchstructures often are described as molecular sieves, and the volumetricamount of components that may be adsorbed by a molecular sieve is termedthe molecular sieve capacity.

In an adsorption process, separation of the fluid components may beaccomplished because the material may have a physical attraction for oneor more of the components of the mixture in preference to othercomponents of the mixture. Although all of the components of a mixturemay be attracted in varying degrees to the material, there is apreference engineered into the process, such that predominantly thedesired component(s) may be attracted and remain with the material inpreference over all others. Therefore, even if less preferred componentsof a mixture initially come into contact with a portion of the material,because of the stronger attraction of the material for the desiredcomponent(s) of the mixture, the less preferred component(s) may bedisplaced from the material by the desired, and more strongly preferred,component(s). Although the fluid mixture entering a sieve chamber mightbe composed of multiple components, the fluid mixture initially leavingthe vessel would be composed largely of the components which had beenless preferentially adsorbed into the material.

In adsorption processes using adsorbent solids, separation occurs for aperiod of time, but eventually all the available surface sites on and inthe solids are taken up by the desired component(s) or are blocked byconcentrations of unwanted components. At that point, little significantadditional adsorption of component(s) from the mixture is likely tooccur, and the fluid mixture which might be withdrawn from the chambermay be insignificantly changed by further exposure to the solids. Theadsorption step of the process is thus ended, and the component(s) whichhave been adsorbed by the solids must be removed from the solids, so asto effect separation and permit reuse of the solids.

A suitable adsorption apparatus or system might first permit adsorptionof a product comprising the desired component(s) by the solids and latertreat the solids to cause them to release the product and permitrecovery of this product. Such an adsorption apparatus or system mightcomprise a “moving-bed” which permits movement of a tray or bed of thesolids through a chamber, such that at different locations, the solid issubjected to different steps of an adsorption process, e.g., adsorption,purification, and desorption. These steps are defined in greater detailbelow. Nevertheless, moving the solids through an adsorption apparatusmay be difficult and involve complex machinery to move trays or beds. Italso may result in loss of the solids by attrition. To avoid theseproblems, some adsorption apparatus and systems have been designed to“simulate” moving the tray(s) or bed(s) to the locations, e.g., zones,of different steps of an adsorption process. Simulation of the movementof the tray(s) or bed(s) may be accomplished by use of a system ofconduits which permits directing and redirecting the streams of fluidsinto the chamber at different zones at different times. As these streamchanges occur, the solids are employed in different steps in anadsorption process as though the solids were moving through the chamber.

The different zones within an adsorption apparatus or system are definedby the particular step of the adsorption process performed within eachzone, e.g., (1) an adsorption step in the adsorption zone; (2) apurification step in the purification zone; (3) a desorption step in thedesorption zone. A more detailed explanation of the zones of theadsorption process follows.

Adsorption Zone: when a multicomponent fluid feedstream, such as afeedstream comprising the C8 aromatics orthoxylene (OX), metaxylene(MX), paraxylene (PX), and ethylbenzene (EB), is fed into the adsorptionapparatus or system, the portion of the apparatus or system into whichthe feedstream is being fed is termed an “adsorption zone.” In theadsorption zone, the fluid comes into contact with the adsorbentmaterial, and the desired component(s) are adsorbed by the adsorbentmaterial. As noted above, other components may also be adsorbed, butpreferably to a lesser extent. This preferential adsorption may beachieved by the selection of an adsorbent material, e.g., adsorbentsolids, which have a preference for adsorbing the desired component(s)from the multicomponent feedstream. Although only the desiredcomponent(s) may have been adsorbed by the solids, other lesspreferentially adsorbed components of the fluid mixture may still remainin void spaces between the solids and possibly, in the pores, channels,or holes within the solids. These unwanted components preferably areremoved from the solids before the desired component(s) are recoveredfrom the solids, so that they are not recovered along with the product.

Purification Zone: after adsorption, the next step is to purify thefluid and adsorbent material in the chamber. In this step, the tray(s)or bed(s) may be moved or flow within the conduits may be changed, sothat the multicomponent feedstream may no longer be fed into theadsorption zone. Although the tray(s) or bed(s) have not physicallymoved, the material may now be described as being in a “purificationzone” because a fluid stream, e. g., a purification stream, is fed intothe adsorbent material to flush the unwanted components from theadsorbent material, e.g., from within and from the interstitial areasbetween the solids. Thus, a fluid comprising unwanted components, i.e.,raffinate, is flushed from the purification zone by substituting a fluidcomprising the desired component(s) or other component(s) deemed to bemore acceptable for the unwanted components. The unwanted components maybe withdrawn in a raffinate stream. Because an objective of theadsorption process may be to separate the product comprising the desiredcomponent(s) from other components which may have nearly the sameboiling point or density as the desired component(s), purification maydisplace, unwanted components and substitute another fluid which can bemore readily separated by other means, e.g., distilled.

Desorption Zone. after the solids have been subjected to thepurification stream, the stream in the conduit(s) may again be changedto introduce a desorbent steam into the chamber to release the product.The desorbent stream contains desorbent which is more preferentiallyadsorbed by the solids than the product comprising the desiredcomponent(s). The desorbent chosen will depend in part upon the desiredcomponent(s), the adsorbent materials, and the ease with which thedesorbent can be separated from the product. Once the desorbent streamhas been introduced to the chamber, the product may be withdrawn fromthe chamber.

Each and every step and zone might be present somewhere in an adsorptionapparatus or system if simultaneous operations are conducted.Nevertheless, the steps may be performed successively or staggered overtime. Further, in some adsorption processes, the unwanted components maybe adsorbed, and the product comprising the desired component(s) allowedto pass through the adsorption apparatus or system. Therefore, the termsraffinate and extract are relative and may depend upon the particularnature of the components being separated, the preference of the solids,and the nature of the apparatus or system. Although in embodiments thepresent invention will be discussed primarily in terms of apparatus andsystems in which the product is adsorbed by the solids, the invention isnot limited to such configurations.

An apparatus suitable for accomplishing the adsorption process of thisinvention is a simulated moving-bed adsorption apparatus. A commercialembodiment of a simulated moving-bed adsorption apparatus is used in thewell-known Parex Process, which is used to separate C8 aromatic isomersand provide a more highly pure paraxylene (PX) from a less highly puremixture. See U.S. Pat. Nos. 3,201,491; 3,761,533; and 4,029,717.

Typically, such an adsorption apparatus is contained in a verticalchamber. Such a chamber may be packed with adsorbent solids, possibly intrays or beds stacked within the chamber. More than one type of solidalso might be used. The chamber also may have the capability to performeach of the above-described steps simultaneously within differentlocations, e.g., zones, in the chamber. Thus, the composition of thefluid in the chamber may vary between zones although there may be nostructures completely separating these zones. This may be achieved bythe use of a serially and circularly interconnected matrix of fluidcommunication conduits including associated means, such as valves andpumps, which permit streams to be directed and redirected into differentzones of the chamber and to change the direction of these streamsthrough the solids within the different zones of the chamber. Thedifferent zones within the chamber may have constantly shiftingboundaries, as the process is performed.

The cyclic advancement of the streams through the solids in a simulatedmoving-bed adsorption apparatus may be accomplished by utilizing amanifold arrangement to cause the fluid to flow in a counter currentmanner with respect to the solids. The valves in the manifold may beoperated in a sequential manner to effect the shifting of the streams inthe same direction as overall fluid flow throughout the adsorbentsolids. In this regard see U.S. Pat. No. 3,706,812. Another means forproducing a countercurrent flow in the solid adsorbent is a rotatingdisc valve by which the streams, e.g., feed, extract, desorbent, andraffinate, and line flush, are advanced cyclically in the same directionthrough the adsorbent solids. Both U.S. Pat. Nos. 3,040,777 and3,422,848 disclose suitable rotary valves. Both suitable manifoldarrangements and disc valves are known in the art. More recently, asystem has been described using dual rotary valves (U.S. applicationSer. No. 61/116097, filed 19 Nov. 2008).

Normally there are at least four streams (feed, desorbent, extract, andraffinate) employed in the procedure. The location at which the feed anddesorbent streams enter the chamber and the extract and raffinatestreams leave the chamber are simultaneously shift in the same directionat set intervals. Each shift in location of these transfer pointsdelivers or removes liquid from a different bed within the chamber. Inmany instances, one zone may contain a larger quantity of adsorbentmaterial than other zones. Moreover, zones other than those discussedabove may also be present. For example, in some configurations, a bufferzone between the adsorption zone and the desorption zone may be presentand contain a small amount of adsorbent material relative to the zonessurrounding it. Further, if a desorbent is used that can easily desorbextract from the adsorbent material, only a small amount of the materialneed be present in the desorption zone in comparison to the other zones.In addition, the adsorbent need not be located in a single chamber, butmay be located in multiple chamber or a series of chambers.

Introducing and withdrawing fluids to the beds may comprise a pluralityof fluid communication conduits, and the same fluid communicationconduit may be used in a first instance to input a feedstream into theapparatus or system and later to withdraw an extract stream. This canresult in reduced product purity due to contamination of the withdrawnproduct. Fluid communication conduits may contain unwanted components,such as residue remaining in the conduit from earlier additions orwithdrawals of streams. This problem may be overcome by employingseparate conduits for each stream or by removing such residue from theconduits by flushing them with a medium which would not effect productpurity as adversely as would a unwanted component remaining in the fluidcommunication conduit. A preferred flushing medium has been the productor the desorbent, which might be more readily separated downstream ofthe chamber than would the residue. See U.S. Pat. No. 4,031,156.Nevertheless, flushing conduits with the product reduces the output ofthe adsorption process.

A standard Parex unit for separating paraxylene (PX) from metaxylene(MX), and orthoxylene (OX) has a single feed to a single rotary valve orparallel rotary valves. The rotary valve directs the feed to a bed line,which is somewhere between the extract (which may comprise, by way ofexample, 99.7% PX and desorbent) and the raffinate (PX-depleted xylenesand desorbent) withdrawal points. Since the UOP Parex process is asimulated moving bed process, the bed lines are shared with all of thefeed and product streams, and therefore the bed lines must be flushedbetween the feed injection point and the extract withdrawal point inorder to prevent contamination of the product.

A standard UOP parex unit has a primary flush which removes the majorityof contaminants and a secondary flush which removes trace impuritiesjust before the extract point. Because the standard UOP Parex unit has asingle feed, various streams of different compositions are typicallyblended together and fed to a single point in the Parex process.

However, as indicated in U.S. Pat. No. 5,750,820, and also U.S. Pat. No.7,396,973, it is better to segregate feeds which are of substantiallydifferent composition, such as concentrated paraxylene from a selectivetoluene disproportionation unit (generally 85-90% paraxylene) andequilibrium xylenes (generally about 23% paraxylene) from a powerformer,isomerization unit or transalkylation unit. This can be done by usingthe primary line flush as a second feed point for the paraxyleneconcentrate and using the secondary flush as the sole flushing stream.Having only a single flush does result in a slight compromise in theseparation process, but the compromise typically is far outweighed bythe benefit of optimizing the feed location of the paraxyleneconcentrate as far as net purity in the final product.

However, there is a problem with the above configuration in that thestandard Parex unit has the secondary flush located close to the extractwithdrawal point in order to minimize contaminants that are withdrawnwith the extract. When the secondary flush is very close to the extractwithdrawal point, the concentrated paraxylene that is being flushed fromthe bed line will be too close to the extract withdrawal point and thehighest separation of the feed will not be realized. This has heretoforenot been recognized.

The present inventors have surprisingly discovered that the feedlocation of both the concentrated paraxylene in the primary flush andalso the location of the secondary flush can be modified to realize thefull benefit of the feed configuration in U.S. Pat. No. 5,750,820. Bymoving the secondary flush further away from the extract, the materialflushed from the bed line will be injected at a more efficient location.In embodiments, improvements associated with less desorbentrecirculation can be realized.

SUMMARY OF THE INVENTION

The invention is directed to a process for separating a product from atleast two multicomponent feeds to an apparatus or system for thecontinuous simulated countercurrent adsorptive separation ofhydrocarbons, and to the apparatus or system for accomplishing the same.

In embodiments, the process comprises feeding at least two differentfeeds, the feeds characterized by having different concentrations of atleast one product, preferably a C8 species selected from one or moreisomers of xylene, or the feeds may be characterized as having adifferent desired product, for instance paraxylene as the desiredproduct of a first feed and orthoxylene as the desired product of asecond feed. It will be recognized by one of skill in the art that acontinuous simulated countercurrent adsorptive separation system canhave many desired end products, such as pharmaceuticals, fragrances,sugars, and the like.

In embodiments, the feed location of both the primary flush and also thesecondary flush are altered as compared with the prior art to realizethe fullest benefit of the present invention.

In embodiments the conduits used to supply the feedstream to theapparatus or system are flushed with media of multiple grades.

In embodiments, the process achieves improvements in one or more ofefficiency of adsorption separation, capacity of adsorption apparatussystems, and purity of product attainable by adsorption process.

In one embodiment, the process comprises the steps of: (a) introducing afirst multicomponent feed, comprising at least one desired product,through at least one fluid communication conduit into a simulatedmoving-bed adsorption apparatus comprising at least one rotary valve andplural sieve chambers; (b) flushing the at least one conduit in step (a)with a sufficient quantity of at least one initial flushing mediumcomprising the at least one desired product in step (a) in an initialconcentration, such that feed residue is flushed from the at least oneconduit in step (a) into the apparatus by the at least one initialmedium; (c) flushing the at least one conduit in step (a) with asufficient quantity of a second and preferably final flushing medium,wherein said second flushing medium is selected from the groupconsisting of desorbent, such as paradiethylbenzene (PDEB), toluene, ora mixture thereof, and a flushing medium comprising the at least onedesired product in step (a) in a final concentration, such that thefinal concentration is not less than and is optionally greater than theinitial concentration and such that initial medium residue from the atleast one initial medium is flushed from the conduit in step (a) intothe apparatus by the final medium; (d) withdrawing the at least onedesired product in step (a) from the apparatus; (e) introducing a secondmulticomponent feed comprising at least one desired product, which maybe the same or different from the at least one desired product in step(a) and which, if the same, is present in the second multicomponent feedin a concentration different from the concentration of the at least onedesired product in said first multicomponent feed, through at least onefluid communication conduit into the apparatus; (f) flushing the atleast one conduit in step (d) with a sufficient quantity of at least oneinitial flushing medium (which may be the same or different from theflushing medium in step (b)) comprising the at least one desired productin step (e) in an initial concentration, such that feed residue isflushed from the at least one conduit in step (e) into the apparatus bythe at least one initial medium in step (f); (g) withdrawing the atleast one desired product in step (d) from the apparatus.

In embodiments, the quantity of the initial medium may not be less thanthat sufficient to flush the feedstream residue from the conduit.

In embodiments, the apparatus comprises plural sieve chambers containingone or more adsorbent material selected from the group consisting ofcharcoal, ion-exchange resins, silica gel, and the like, and thequantity of the initial medium may be sufficient to fill the apparatusto the sieve chamber capacity.

In embodiments, the process includes additional steps including one ormore of flushing one or more conduits with a sufficient quantity of afinal (or third) flushing medium comprising the at least one desiredcomponent in a final concentration, such that the final concentration isgreater than the initial concentration and greater than the secondconcentration, and such that initial medium residue from the at leastone initial medium is flushed from the conduit into the system by thefinal medium; withdrawing a raffinate stream from the system;introducing a desorbent stream to the system; withdrawing a combinationcomprised of the product and the desorbent from the system; and removingthe product from the combination.

In yet another embodiment, the initial concentration of the at least oneinitial medium is continuously increased during the flushing of the atleast one conduit until the initial concentration equals the finalconcentration. Preferably, this may be accomplished by adding theproduct to the at least one initial medium in gradually increasingamounts and decreasing proportionately flow from the source of the atleast one initial medium.

As is well-known per se in the commercial Parex unit, moving thelocations of liquid input and output is accomplished by a fluiddirecting device known generally as a rotary valve which works inconjunction with distributors located between the adsorbent sub-beds.The rotary valve accomplishes moving the input and output locationsthrough first directing the liquid introduction or withdrawal lines tospecific distributors located between the adsorbent sub-beds. After aspecified time period, called the step time, the rotary valve advancesone index and redirects the liquid inputs and outputs to thedistributors immediately adjacent and downstream of the previously useddistributors. Each advancement of the rotary valve to a new valveposition is generally called a valve step, and the completion of all thevalve steps is called a valve cycle. The step time is uniform for eachvalve step in a valve cycle, and is generally from about 60 to about 90seconds. A typical process contains 24 adsorbent sub-beds, 24distributors located between the 24 adsorbent sub-beds, two liquid inputlines, two liquid output lines, and associated flush lines. In anembodiment of the present invention, an improvement is provided wherebythe rotary valve is replumb so that the input of the secondary flush isa least one and preferably two valve steps downstream of where it is,heretofore, ordinary inputted. This is more fully illustrated by thedescription of FIG. 1 below. This also means that the secondary flush isadded closer in sequence to the input of the primary flush, such aswithin three cycle steps.

It is an object of the invention, in one or more embodiments, toincrease the efficiency of adsorption apparatus or systems, wherebycontaminants, such as feedstream residue, may be removed from fluidcommunication conduits by flushing them from the conduits into theapparatus or system with flushing media containing concentrations of thedesired component(s) of the product which are higher than that of thefeedstream. It is an advantage of such embodiments that if the productis extracted through the same conduits that carried the feedstream, suchas in a simulated moving-bed adsorption apparatus, extract will not becontaminated, or will have lower contamination, with feedstream residue.

It is an additional object of this invention to increase the capacity ofan adsorption apparatus or system. It is an advantage of this processthat excess capacity of the apparatus or system may be more fullyutilized by purifying the solids with flushing media and flushingconduits with media containing the desired component(s). It is a featureof such embodiments that fluid communication conduits may be flushedwith media containing concentrations of a desired component orcomponents higher than that of the feedstream, which may be drawn from asource other than the apparatus.

It is yet another object of this invention, in embodiments, to increasethe purity of the product obtained from an adsorption apparatus orsystem. It is a feature of embodiments of this process that contaminantsmay be removed from conduits and from pores, channels, and holes inadsorbent solids, and conduits may be charged with the product. It is anadvantage such embodiments that the product may be recycled through theapparatus or system, and excess apparatus or system capacity may be usedto further separate other unwanted components of the feedstreamremaining in the product.

These and other objects, features, and advantages will become apparentas reference is made to the following detailed description, preferredembodiments, examples, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawing, like reference numerals are used to denotelike parts throughout the several views.

FIG. 1 is a schematic illustrating a comparison of the prior art with anembodiment of the invention.

DETAILED DESCRIPTION

According to the invention, there is provided a process for separating aproduct from at least two multicomponent feeds to an adsorptionapparatus or system. The apparatus or system may comprise a moving-bedor a simulated moving-bed adsorption means, and in embodiments providesa product comprising at least one organic compound, such as an arylcompound with alkyl substitutes. In embodiments the conduits used tosupply the feedstream to the apparatus or system are flushed with mediaof multiple grades. In embodiments the process achieves improvements inone or more of efficiency of adsorption separation, capacity ofadsorption apparatus systems, and purity of product attainable byadsorption process.

In embodiments the feed location of both the concentrated paraxylene inthe primary flush and also the location of the secondary flush arelocated to realize the full benefit of feed locations. In embodiments,by moving the secondary flush further away from the extract, thematerial flushed from the bed line will be injected into a moreadvantageous point in the profile. This allows for additional capacityor decreased use of energy associated with a decrease of desorbentrecirculation will be realized.

The process of an embodiment of the present invention employs thesimulated countercurrent flow processes such as described in U.S. Pat.Nos. 3,201,491; 3,761,533; and 4,029,717. The diagram in FIG. 1 will beunderstood by those of skill in the art to depict a simulated moving bedprocess. Desorbent is introduced through conduit 100, flush leaves theapparatus through conduit 101, extract (containing the desired product)leaves the apparatus via conduit 102, raffinate leaves the systemthrough conduit 110, the secondary flush is added through conduit 103,the primary flush is added through conduit 106, a first multicomponentfeed is added to the system through conduit 107 and a secondmulticomponent feed is added through lines 108 or 109, as explained morefully in the following description.

In FIG. 1, the adsorbent 112 moves upward through the sieve chambervessel 120 containing plural bedlines A1 through An+j. The hydrocarbonliquid feed 111 flows countercurrent to the circulating adsorbent. Inoperation, the adsorbent does not flow, but the various feed and productstreams cycle through the bed lines, represented by lines A₁ throughA_(n+j), at a rate that is different than the circulating hydrocarbon.This simulates the movement of the bed lines A₁ through A_(n+j).Theoretically there may be any number of bed lines, thus n>2 and n+j isthe maximum number of bedlines, however from a practical standpoint thenumber of bed lines is limited by design considerations and otherfactors. A further discussion may be found in the prior art. What isimportant is the relative positions of the bedlines caused by thestepping of the rotary valve, as would be understood by one of skill inthe art (such as that n and j are positive integers and that in typicalcommercial embodiments the total number of bedlines is 24). Certainbedlines, i.e., bedlines between A₂ and A_(n), bedlines A_(n+3),A_(n+5), A_(n+6), and A_(n+10) through A_(n+j−1) are not depicted inFIG. 1, for convenience of view.

In a conventional unit, the sieve preferentially starts adsorbing theparaxylene molecules in the feed 107 and flows upward. In embodiments,the feed is selected from the group consisting of equilibrium xylenesand ethylbenzene (which is about 21-24 wt % PX), concentrated PX from anSTDP unit (which is about 85-90 wt %), and admixtures thereof.

The paraxylene is desorbed from the sieves in the bedlines by desorbentstream 100, the main component of which also is strongly adsorbed on thesieve(s) in bedlines A₁ through A_(n+j), but has a different boilingpoint and is easily separated from the desired product(s) downstream ofthe apparatus. In embodiments, the desorbent is paradiethylbenzene(PDEB), toluene, or a mixture thereof, or some other strongly adsorbedcompound.

The extract 102, which in the embodiment described is a mixture of thepurified paraxylene and the desorbent, is withdrawn at a point betweenthe feed 107 and the desorbent 100. The raffinate 110, which consist ofthe paraxylene-depleted (less strongly adsorbed) xylenes and desorbent.

Because this is a simulated moving bed process, the various feeds andproducts must share the lines between the bedlines (sieve beds) androtary valve (not shown). To prevent loss of paraxylene molecules in thehydrocarbon recycle 111 and subsequently the raffinate 110, the bedlines between the extract out 102 and desorbent in 100 are flushed 101.The flush out can either be sent to the extract tower for recovery orrecycled and used for primary flush in 106.

In addition (and more importantly), since feed 113 is routed through thetransfer lines (not shown) between the rotary valve (also not shown) andthe sieve chambers A₁ through A_(n+j) before extract 102 the transferline should be thoroughly flushed to avoid contamination of the productextract 102. Either finished paraxylene product or desorbent, e.g., PDEBor toluene, are routed through a primary flush 114 and a secondary flush104. The secondary flushing step is just after the extract withdrawallocation 102 in order to flush any trace amounts of contaminants thatmay have leaked from the sieve chamber(s) back into the bed lines.

As taught in U.S. Pat. No. 5,750,820, it is an improvement to segregateconcentrated paraxylene, such as may be obtained downstream from 102 bydistillation and then recycle, and use it as primary flush 114 in placeof location 106. This step is beneficial because it routes theconcentrated paraxylene into a more optimum place in the compositionprofile. In addition, while the concentrated paraxylene 109 is not aspure as desorbent 100 or product paraxylene (extract 102 is acombination of product para-xylene and desorbent, which can be separateddownstream such as by distillation), it does reduce the amount ofcontaminants in the bed lines A₁ through A_(n+j) and facilitate thesecondary flushing step.

However, flushing a bed line (i.e., A₁ through A_(n+j)) full ofconcentrated PX (85-90 wt %) right next to the extract creates a problemwhich was unanticipated and not even recognized by the inventors of theaforementioned improvement. Without wishing to be bound by theory, whathappens is that you are now flushing in a material that has about 10 vol% impurities when you are trying to get down to 2-3 wt % impurities.

The present inventors, having discovered the problem, have now alsodiscovered that in order to realize the full benefit of the movement ofthe input of the concentrated stream in line 108 from 107 to 106 vialine 109, the feed location of the secondary flush 103 must be moved toan improved place in the composition profile, e.g., further from 104,and closer to 113. One embodiment of such is depicted in FIG. 1 as line105. Again, it should be emphasized, as would be known by one of skillin the art, that these positions are relative and that, although theactual positions change by virtue of the movement of the rotary valve(not shown), the relative positions of the lines remains the same.

It will be understood by one of ordinary skill in the art that FIG. 1depicts a simplified simulated moving-bed apparatus with a rotary valve,wherein countercurrent “movement” of the solids in bed lines A₁ throughA_(n+j) relative to the fluid streams is simulated by the use of therotary valve, which is not shown in the figure. As the valve rotates,the zones previously discussed move through the column in a stepwisesequence due to the change in the stream flows through the valve. Inembodiments, a preferred rotary valve for performing this invention isdescribed in U.S. Pat. No. 3,205,166, the disclosure of which isincorporated herein by reference. In this arrangement, each fluidcommunication conduit connected to the chamber may serve a differentfunction with each step rotation of the rotary valve.

Trade names used herein are indicated by a ™ symbol or ® symbol,indicating that the names may be protected by certain trademark rights,e.g., they may be registered trademarks in various jurisdictions. Allpatents and patent applications, test procedures (such as ASTM methods,UL methods, and the like), and other documents cited herein are fullyincorporated by reference to the extent such disclosure is notinconsistent with this invention and for all jurisdictions in which suchincorporation is permitted. When numerical lower limits and numericalupper limits are listed herein, ranges from any lower limit to any upperlimit are contemplated.

What is claimed is:
 1. An apparatus for the continuous simulatedcountercurrent adsorptive separation of hydrocarbons comprising pluralsieve chambers fluidly connected with plural bedlines A1 through An+j,wherein n>2 and n+j is the total number of bedlines, which in turn arefluidly connected with a matrix of fluid communication conduits todistribute plural input streams, including desorbent, a primary flushinput and a secondary flush input, and at least two multicomponent feedsdiffering in the concentration of paraxylene, to said plural bedlines,and plural output streams, including a primary flush output and asecondary flush output, an extract comprising a paraxylene-enrichedproduct, and raffinate, from said plural sieve chambers, the improvementcomprising: when bedline An contains extract, bedline An+9 contains afirst multicomponent feed, bedline An+7 contains a second multicomponentfeed, then said secondary flush input is contained in a bedline betweenbedlines An+2 and bedline An+7.
 2. The apparatus of claim 1, wherein thefirst multicomponent feed has a concentration of paraxylene of at leastabout 23 wt % and the second multicomponent feed has a concentration ofparaxylene above about 80 wt %.
 3. The apparatus of claim 1, wherein thesecondary flush comprises about 100 wt % paradiethylbenzene.